1. Field of the Invention
The present invention relates to an intermittent ozone feeding apparatus.
2. Description of the Prior Art
Ozone has a remarkable oxidizing effect and does not cause public polution. Accordingly, ozone has been used in various fields for environmental treatment and treatment in chemical industries. A continuous ozone treatment process and an intermittent ozone treatment process have been employed depending upon the purposes of application of ozone. An intermittent ozone treatment has been utilized for preventing deterioration of the functioning of a coolant water pipe equipped in instrumentation of a power plant, a chemical factory or a machining factory which is caused by adhesion of living organisms such as algae and shell-fish causing deterioration of the heat exchange efficiency and clogging of the pipe or for preventing various problems in a water quality tester or other instrument caused by adhesion of algae and shell-fish in a clean water passage or a waste water passage. In the treatment, ozone is intermittently fed one to several times for several minutes per feeding, each one to several days to inhibit propagation of the living organisms.
If an ozonizer is intermittently operated during the intermittent application of ozone, a large sized ozonizer is required, causing higher instrumention costs. Usually, an intermittent ozone feeding apparatus generates ozone by a smaller ozonizer which is stored in a silica gel at a low temperature for a relatively long time period (one to several days) and the ozone is desorbed once for several minutes to feed ozone into the water for treatment.
FIG. 1(a) is a diagram of a conventional intermittent ozone feeding apparatus and FIG. 1 (b) is a sectional view of an adsorption-desorption tower taken along a perpendicular plane. In FIG. 1(a), reference number (1) designates an ozonizer; (2) designates an adsorption-desorption tower into which an ozonized oxygen is fed from ozonizer (1); (3) designates a recycling blower for recycling the oxygen from adsorption-desorption tower (2) to the ozonizer (1); (4) designates an oxygen feeding source to ozone generator (1); (5a) (5d) designate, respectively, electromagnetic valves; (6) designates a hot brine tank for receiving brine from the adsorption-desorption tower (2); (7) designates a heater equipped with the hot brine tank; (8) designates a pump for feeding the brine to adsorption-desorption tower (2); (9) designates a refrigerator for cooling adsorption-desorption tower (2); and (10) designates a water ejector for suction of ozone from adsorption-desorption tower (2).
In FIG. 1(b) reference number (2a) designates an ozone adsorbent packed in adsorption-desorption tower (2) and is usually made of silica gel. Reference number (2b) designates an inner column for holding the ozone adsorbent; (2c) designates an outer column; (2d) designates an adsorption-desorption brine tank formed between the inner column and the outer column; and (2e) designates an evaporation pipe which closely contacts with inner column (2b) and is connected to refrigerator (9).
The operation of the apparatus will now be discussed referring to the operation timing sequence shown in FIG. 2. The operation is classified into an ozone adsorption and an ozone desorption period. The arrowed lines indicate operation timings of the instruments. In the case of an electromagnetic valve, the arrowed line indicates the opening state.
The ozone adsorption process will now be described in detail. An oxygen recycling system is formed by ozonizer (1), the adsorption-desorption tower (2) and the recycling blower (3) in that order. Electromagnetic valves (5a), (5b) are opened and electromagnetic valves (5c), (5d) are closed. Oxygen is fed from oxygen feeding source (4) at a constant pressure (usually 2 ata) in the system. The ozonized oxygen formed by ozonizer (1) is fed into adsorption-desorption tower (2) for ozone to be adsorbed by ozone adsorbent (2a). Oxygen (95% or more) which is not ozonized by ozonizer (1) is recycled by recycling blower (3) into ozonizer (1) to form the oxygen recycling system. The tower is cooled to lower than -30.degree. C. by refrigerator (9) during the ozone adsorption period, since the amount of ozone adsorbed in the tower (2) is higher with respect to lowering of the temperature of the silica gel.
Usually, cooling is carried out by evaporating Freon compressed by refrigerator (9) in evaporation pipe (2e) which closely contact with inner column (2b). Thus, ozone is adsorbed in adsorption-desorption tower (2). When the concentration of ozone on ozone-adsorbent (2a) reaches a nearly saturated concentration after a predetermined time, leakage of ozone from the gas outlet of adsorption-desorption tower (2) is initiated. If the adsorption operation is continued after initiating the leakage of ozone, loss of electric power is caused. Therefore, the operation for adsorption is discontinued and desorption is started. The period for the desorption is for predetermined length of time.
The ozone desorption will now be described in detail. In desorption of ozone, electromagnetic valves (5a), (5b) are closed, electromagnetic valves (5c), (5d) are opened, water is fed into water ejector (10) to suck ozone under a reduced pressure from adsorption-desorption tower (2) and ozone is dissolved to obtain ozonized water. At the same time, pump (8) is actuated whereby brine is fed from hot brine tank (6) heated by heater (7) usually at 50.degree. C. into adsorption-desorption brine tank (2d) and ozone adsorbent (2a) cooled during the operation for adsorption to the low temperature is heated to accelerate the desorption of ozone.
The operation for adsorption of ozone is performed during a relatively long period such as, for example, one to several days whereas the operation for desorption of oxygen is performed for a relatively short time of one to several minutes by heating under a reduced pressure in the adsorption-desorption tower (2). After desorption, the adsorption operation is started again by feeding oxygen into the system from oxygen feeding source (4) and cooling adsorption-desorption tower (2) by refrigerator (9).
In the conventional intermittent ozone feeding apparatus, when ozone is introduced into a water pipe system there is a possibility that the water will flow reversely so as to enter into adsorption-desorption tower (2) because tower (2) is desorbed at a reduced pressure (less than atomospheric pressure). If the water flows reversely into adsorption-desorption tower (2), the silica gel, having adsorbed a large amount of ozone and oxygen, undergoes an abrupt decomposition or desorption of ozone so as to possibly cause an explosion. Even if an explosion is not caused, it is necessary to replace the silica gel with new gel because moisturized silica gel can not adsorb ozone. Intermittent ozone feeding apparatuses are, therefore, needed to prevent the reverse flow of water into the adsorption-desorption tower (2).
Reverse-flow of water into the silica gel in the conventional apparatus and an attempt to prevent it will now be described with reference to FIGS. 1 to 3. In these figures, upon completion of ozone desorption period, the pressure in adsorption-desorption tower (2) is usually at 0.1 ata which provides a condition sufficient to cause a reverse flow of water in a water pipe during an oxygen packing period. The leakage of water can, however, be prevented if electromagnetic valve (5c) is closed. Since the conventional electromagnetic valve has a directional function which stops flow in one direction, two electromagnetic valves (5c1), (5c2) connected in an opposite direction are usually used. Electromagnetic valve (5c1) is mainly used for preventing leakage of the ozone and the oxygen out of the system during an ozone adsorption period and electromagnetic valve (5c2) is used for preventing leakage of the water (or air outside the system) in to tower (2) during the time period between completion of the ozone desorption period and filling of the system with oxygen. Conventional electromagnetic valve (5c) is normally of a non-leaking type electromagnetic valve which provides complete closing of the gas or liquid when closed and which is very expensive and has a short life span. Despite the use of two such electromagnetic valves (5c1), (5c2) to prevent reverse flow of the water into adsorption-desorption tower (2), there is still a risk of reverse flow of the water during the oxygen packing period in the operation timing sequence shown in FIG. 2 if the valve body of electromagnetic valve (5c) becomes clogged with foreign matter so as to prevent complete closing of the valve. During the ozone adsorption period which is a major portion of the operational time of the conventional apparatus, a water leaking condition is established so that water may enter into the silica gel even with little leakage.
Another problem exists with the conventional intermittent ozone feeding apparatuses. An oxygen receiver is normally used as an oxygen source (4) for a small sized apparatus. The receiver contains a water content of about 1000 ppm when the dew point of the oxygen is about -20.degree. C. Almost all water is adsorbed by the silica gel packed in adsorption-desorption tower (2) since silica gel is a strong adsorbent. Thus, when the silica gel adsorbes water, the ozone adsorption capacity of the silica gel decreases and ozone is decomposed whereby the amount of adsorbed ozone, i.e. desorbed ozone decreases depending upon the adsorption of water. In the conventional apparatus, it is necessary to periodically replace the silica gel in order to obtain normal operation of the apparatus. This requires a great deal of labor and a high cost. It is also necessary to stop operation of the apparatus for several days in order to replace the silica gel. Furthermore, living organisms have been found on the inner surface of the pipes during discontinuation of operation.